It's healthy to be skeptical. But it's unhealthier not to try everything and discover what works. It's pretty rare for someone to immediately see which of several possible new inventions might be better (and why they're better). I'm referring to the inventor -- an inventor often has several possibilities for new things he could try to do, and he has to choose which of those to pursue. It's not at all obvious which path to choose. So most of the time, you just try all possibilities as fast as possible, letting intuition guide you. For example, it probably wasn't immediately obvious to DHH that it was a win to write websites in Ruby until he'd tried writing websites in Ruby to see how it'd turn out. History tends to get rewritten so that it's obvious in hindsight that it would work. But it wasn't at all obvious until it was suddenly obvious to him, which was only after he'd had the experience of trying it.

So the interesting aspect here is that Carmack's intuition has told him that there might be something worthwhile for the videogame industry to look into functional programming languages. It probably won't pan out. But if there's any way it can, then Carmack will find out how to make it a pragmatic way for studios to build large codebases.